When Justice John Paul Stevens underwent confirmation hearings for his nomination to the Supreme Court in 1975, he was not even asked about his stand on Roe v. Wade. Gone is this era of unpoliticized nominations proceedings, and there are now a myriad of political considerations that will need to go into the choice of nominee. That said, while there will be hype and hoopla in the weeks to come, the fact is that President Obama has no chance of altering the ideological balance of the Court in his favor and can only attempt to minimize the losses associated with exchanging the most senior liberal justice on the bench with the most junior one. Since Stevens has stood reliably with three other liberal justices in the Court since the mid-90s, Obama will at best succeed only in maintaining the status quo in which the right-leaning Justice Anthony Kennedy often casts the deciding vote. More likely than not, Obama would have to settle on a nominee who will be considerably less liberal than Stevens, because we are now in a very different era from when Stevens was confirmed by a vote of 98-0 after 19 days of hearings in 1975.
Conservatives, on the other hand, have a lot to gain if they bring in a fifth conservative Justice, perhaps under cover of a Trojan Horse (which some say - inaccurately, I think - Justice Stevens was in 1975). That Stevens is now characterized as the liberal lion in the Court is evidence enough of the Reagan jurisprudential Revolution. When Stevens was appointed by Gerald Ford, he was a centrist Republican and often the maverick who did not agree with colleagues on either ideological side. With his retirement, the Reagan Revolution in the Court would be complete, with the majority of the Court now having been appointed by Republican Presidents Reagan, Bush 1 and Bush 2.
In order to minimize liberal losses, Obama will likely do two things: pick a young justice, and one further to Left on one or two key liberal issues which are relatively orthogonal to Tea Party concerns and therefore will not precipitate an intense outcry.
1. The retirement of John Paul Stevens unquestionaby weakens the liberal block in the Court because even if the new Justice were as reliably liberal as Stevens was, s/he would not be as senior. With the departure of the most senior member of the Court who also happens to be liberal, the liberal block in the Court will lose the agenda-setting power of their "ranking" member who, when the Chief Justice is not in the majority, typically decided which of the Associate Justices would write the majority's opinion. The next most senior Justice on the Court following Stevens is the arch-conservative Antonin Scalia, appointed by Reagan. Seniority also matters because during their private conferences, Justices speak and vote according to seniority. Liberals would lose an experienced jurist who has spent 34 years on the Court exploiting his seniority and skills to build majority coalitions with the inclusion of the occasional conservative justice. And unfortunately for liberals, the two justices appointed by Bill Clinton, Ruth Bader Ginsburg and Stephen Breyer, are also among the oldest on the Court. If Obama is thinking about locking in his legacy, he will note that for the average age of the liberal judges to be equal to the average age of the conservative judges, his nominee will need to be 57 years old.
2. On the brighter side for Obama will be the realization that this battle is his to lose. The Senate has historically endorsed the president's nomination, though there are 12 exceptions to this, with Reagan's nominee, Robert Bork, being the most recent example. (President Bush withdrew Harriet Myers' nomination before it came to a vote.) The Senate has never filibustered the nomination of an Associate Justice, though the Republicans did launched a filibuster against the promotion of Associate Justice Abe Fortas to Chief Justice in 1968. Democrats tried to filibster Samuel Alito's confirmation, but then ultimately voted 72-25 to invoke cloture. If even Sonia Sotomayor, who had made certain controversial remarks about the wisdom of a latina over white male judges ended up with a positive vote of 68-31, then Obama is likely to win the nomination battle. Knowing this then, he could probably afford to go more to the Left to fill a vacancy created by the most liberal justice presently sitting on the Court. Liberal groups will insist on someone who would fill Stevens' ideological shoes, so Obama must and probably will appease them to some degree. But he would be wise to take heed of rise of the libertarian faction within the conservative movement (in the Tea Party Movement and the strong showing of Ron Paul in the CPAC straw poll and in the Southern Republican Leadership Conference straw poll on Saturday) to know to which part of the far / further Left he should not venture when picking his nominee.